The present invention relates to a flaw detecting device, and more particularly to a device for detecting surface flaws of rapidly moving sheet material.
In particular, the invention relates to a device for detecting flaws on or in the surfaces of rapidly moving metal strip in rolling mills.
In modern rolling mills in which billets of metal are converted into sheet metal bands or tapes, the tapes move at speeds as high as 1600 meters per minute. At these speeds, it is very difficult to detect surface flaws in the strip material before the same is taken up to form strip material coils. Yet, the recognition of such flaws is important and it is therefore known, in order to make even relatively small flaws clearly visible, to illuminate the moving band with high-intensity stroboscopic lamps. These stroboscopic light flashes serve to produce a high intensity of light which increases the chance of identifying the flaws, and the short duration of the flashes eliminates the need for an optical speed shutter (rapidly operating shutter) which otherwise is necessary to obtain sharp-contour viewing when pictures are made of rapidly moving objects for direct observation by the naked eye or else by television or moving picture cameras.
The known devices have certain disadvantages. One of these is that a high flash sequence of at least 50 cycles is necessary, because lower flash sequences result in a flickering which has been found to be painful for even relatively far removed operating personnel. Moreover, the high flash frequency results in a rapid deterioration of the bulb and the high light intensity is found to be tiring to persons operating in the vicinity and may even cause damage to their eyes.
In addition, the use of monochromatic or polarized light for detecting flaws with the prior-art devices is impossible to all intents and purposes, because filters which would have to be placed in front of the flash lamps would filter out approximately 80% of the light and the residual light would yield only an insufficient light intensity. If, on the other hand, the output of the flash lamps is appropriately increased to overcome this problem and to restore the initial light intensity despite the use of the filters, then the currently common and already high output of 1000 Ws would be increased significantly by a multiple, and it would then be necessary to provide expensive cooling devices for the filters. Analogous disadvantages would result if the visible light would be replaced by light in the infrared or ultra-violet range.